THE ORGANS OF THE INNER GERM-LAYER. 295 



II. Differentiation of the Alimentary Tube into Separate Regions 

 and Formation of the Mesenteries. 



At first the alimentary tube is broadly in contact (fig. 116) with 

 the dorsal wall of the trunk ; it is united to the chorda (ch), the 

 neural tube, and the primitive segments by means of a broad tract of 

 embryonic connective tissue, in which the fundaments of two large 

 blood-vessels, the primitive aortse (ao), are enclosed. The right and 

 left portions of the body-cavity are therefore still separated from 

 each other on the dorsal side by a considerable distance. The older 

 the embryo is, the less this distance becomes, until there results a 

 mesentery, a structure which is established along the whole length of 

 fEeTmtestinal tube, with exception of the anterior portion, in the 

 following manner (compare, Plate I., figs. 8 and 9 with fig. 10). The 

 alimentary tube recedes from the chorda ; at the same time the broad 

 tract of connective tissue previously mentioned becomes narrower 

 from right to left, but elongated dorso-ventrally (fig. 10, Plate I.) ; 

 the two aortse embraced in it move nearer arid nearer together and 

 finally fuse into a single trunk, which lies in the median plane between 

 cEordaTand intestine. After the further advance of this process the 

 alimentary tube and chorda remain united by means of only a thin 

 band, which stretches from the front to the hind end of the embryo. 

 This proceeds from the connective tissue enveloping the chorda, 

 encloses along its line of origin the aorta, and is composed of three 

 layers : a connective-tissue lamella, in which blood-vessels run to 

 the intestine, and two epithelial coverings, which are derived from 

 the middle germ-layer and are now composed of greatly flattened 

 cells. 



The differentiation of the alimentary tube into separate non-equivalent 

 regions lying one behind the other begins with the development of the 

 stomach. This first becomes distinguishable, at some distance be- 

 hind the respiratory tract, as a small spindle-shaped enlargement, the 

 long axis of which corresponds with that of the body (figs. 162 and 

 163 Mg). Such a condition is attained by the human embryo of the 

 fourth week. Five successive regions may now be distinguished in 

 the whole embryonic alimentary tube : the oral cavity, the throat- 

 cavity with its visceral clefts, which is narrowed into the shape of a 

 funnel where it merges into [the third region,] the gullet. This is 

 followed by the spindle-shaped enlargement, the stomach, and the 

 latter by the remaining portion of the alimentary tube, which still is 

 more or less broadly connected (Ds) with the yolk-sac. Excepting 



